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What is Body Mass Index (BMI) and Can I Rely on It?

A frequently used index to assess a person's body composition (or amount of body fat) is called the body mass index, or BMI. This assessment compares your body weight to your height to come up with a value that indicates whether you are underweight, normal weight, overweight or obese.

Here is how to calculate a client’s Body Mass Index:

BMI = Weight (kg) / (Height (m)²)

Practical example: An individual who is 5'7'' in height and weighs 150 pounds

Understanding your body composition is valuable due to the direct correlation between body fat and health. For example, those with a BMI >30 are classified as obese, which can lead to a number of diseases such heart disease, type 2 diabetes, hypertension and stroke. The BMI measurement is also important for Cardiovascular Risk Stratification. When completing a CVD Risk Factor assessment, an individual who has a BMI >30 will score a positive score toward CVD risk Stratification. This can put the individual at a higher risk for moderate to vigorous exercise. Working toward weight loss if you are overweight or obese can have a profound positive effect on your overall health. The table below provides established BMI norms based on various categories of health and fitness.

BMI Reference Chart

Weight Category

BMI Range

Underweight

<18.5

Normal Weight

18.5-24.9

Overweight

25.0-29.9

Grade I Obesity

30.0-34.9

Grade II Obesity

35.0-39.9

Grade III Obesity

>40

*Source: ACE Health Coach Manual page 300 table 11-5

For most people, BMI is a quick and easy way to assess body composition, which is why it’s so commonly used. However, since body weight (and not percent body fat) is used in determining BMI, it may not be the best way to assess those who are heavily muscled (such as body builders) or who have an athletic body type. Because muscle weighs more than fat, those who have well-developed muscles typically appear overweight or obese according to the BMI reference chart. We do not recommend using BMI as the only method to assess your body composition for those with an athletic of muscular build. An estimation of body fat using the skinfold method would be a more accurate choice.

By Brian Greenleebrian.greenlee@acefitness.org

Brian Greenlee, MBA is the National Accounts Manager for the American Council on Exercise. He holds a Masters in Business Administration, a dual Bachelor’s degree in Sports Management and Business Administration, and has earned all four ACE Certifications. Brian has worked on the business and sales side for professional sports teams including NBA, NHL, MLB, WNBA and division I collegiate athletics.